Your nose makes a gas that your mouth cannot
James Nestor spent years interviewing physiologists, free divers, and pulmonologists. One finding kept surfacing: the nasal cavity does something the mouth simply cannot replicate.
The sinuses and nasal passages produce nitric oxide, a signaling molecule that acts as a vasodilator — it widens blood vessels and helps hemoglobin release oxygen more efficiently to tissues. Breathing through the nose delivers that nitric oxide directly into the lungs with each inhale, while breathing through the mouth bypasses the production site entirely.
James Nestor, whose book Breath surveys decades of pulmonology and clinical breathing research, summarizes estimates suggesting nasal breathing can increase oxygen absorption by roughly 18 to 20 percent compared with mouth breathing — a difference that accumulates over the course of a day.
The production of nitric oxide also has immune significance. Nitric oxide is antimicrobial, helping to disable some bacteria and viruses before they reach the lower respiratory tract.
The mechanism is simple enough to demonstrate: breathe slowly through your nose for a few minutes, then through your mouth for the same duration. The sensations in the chest and throat are noticeably different. What the nose does in warming, filtering, and chemically enriching each breath is not cosmetic — it is part of how the lung was designed to receive air.
Mouth breathing during sleep, which many people do habitually, short-circuits all of this, and a growing body of research links chronic mouth breathing with poorer sleep quality and higher rates of sleep-disordered breathing.